The issues presented in this study concern the important problem of the occurrence of cyanobacterial blooms in surface water used for water supply purposes. The objective of this study was to analyze the occurrence of cyanotoxic risk in the catchment area of the Obrzyca River, which is a source of drinking water for the inhabitants of Zielona Góra. At the points of Uście and at its tributary at Wojnowo, the river was more exposed to the blooms of toxigenic algae, whose active microcystins synthesize more here than in other places (Sadowo, Chwalim, Ostrzyce, Lubiatów). Throughout the study, conducted in 2008-12, we observed several times that the amounts of cyanobacteria exceeded 500·10 3 individuals per litre (ind.·dm , was found at Wojnowo in September 2011. The analysis of the physico-chemical water quality indicators showed that the places along the Obrzyca River where periodic cyanobacterial blooms took place were characterized by eutrophic conditions, and the total nitrogen to total phosphorus ratio (N/P) fell within the range of 10 to 16, and the water temperature exceeded 20ºC. In order to assess the toxic hazard caused by periodic cyanobacterial blooms in the catchment area of the Obrzyca River for drinking water for Zielona Góra residents, regular monitoring of the cyanobacteria and their toxins is required -especially at the points in Uście and Wojnowo as well as at the water intake at the water treatment plant (WTP) in Zawada (in central-western Poland).

Harmful cyanobacteria and their cyanotoxins may contaminate drinking water resources and their effective control remains challenging. The present study reports on cyanobacterial blooms and associated cyanotoxins in the Obrzyca River, a source of drinking water in Poland. The river was examined from July to October 2019 and concentrations of microcystins, anatoxin-a, and cylindrospermopsin were monitored. The toxicity of water samples was also tested using an ecotoxicological assay. All studied cyanotoxins were detected with microcystins revealing the highest levels. Maximal microcystin concentrations (3.97 μg/L) were determined in September at Uście point, exceeding the provisional guideline. Extracts from Uście point, where the dominant species were Dolichospermum flos-aquae (August), Microcystis aeruginosa (September), and Planktothrix agardhii (October), were toxic for Dugesia tigrina Girard. Microcystin concentrations (MC-LR and MC-RR) were positively correlated with cyanobacteria biovolume. Analysis of the chemical indicators of water quality has shown relationships between them and microcystins as well as cyanobacteria abundance.

The research presented in this manuscript concerns the evaluation of the effectiveness of microstrainers, which are designed to reduce the amount of plankton in treated surface water. The efficiency of microstrainer filtration analysis is very important for the proper course of the water-treatment process not only in the Water-Treatment Plant (WTP) in Zielona Góra (central western Poland) but also in other WTPs around the world. The qualitative and quantitative monitoring of the abundance of plankton including cyanobacteria during the particle-filtration process allows not only for the assessment of the potential cyanotoxic risk in surface water providing a source of drinking water, but also allows the evaluation of the action and the prevention of adverse impacts of microstrainers. Over four years of research, it was observed that the largest amount of cyanobacteria before microstrainer filtration took place in May. The dominant species was Limnothrix redeckei. The microstrainer removal of plankton and cyanobacteria was statistically significant. The quantity of removed plankton increased with its increasing content in raw water. The particle-filtration process, by reducing the amount of cyanobacteria, contributes to a decrease in intracellular microcystins.

The issues presented in this study concern a very important problem of the occurrence of cyanobacterial blooms in surface water used for water supply purposes. The objective of this study was to analyze the occurrence of cyanotoxic risk in the catchment area of the Obrzyca River (including Sławskie lake which is the beginning of the river), which is a source of drinking water for the inhabitants of Zielona Góra. In order to evaluate toxicity of cyanobacterial bloom it was conducted toxicological testing using aquatic invertebrates (Daphnia magna, Dugesia tigrina) and heterotrophic bacteria (Escherichia coli, Enterococcus faecalis, Pseudomonas fluorescens). Test samples were collected from May to October, 2012. The most toxic was a sample collected from Lake Sławskie on 20 th October when cyanobacteria bloom with a predominance of Microcystis aeruginosa occurred and the amount of microcystins was the largest. The methanol extract of the sample was toxic only above a concentration of 6·10 3 mg·dm -3 . The lethal concentration (48-h LC 50) for Daphnia magna was 3.09·10 3 and for Dugesia tigrina (240-h LC 50) 1.51·10 3 mg·dm -3 of microcystins (MC-LR, MC-YR and MC-RR). The same extract stimulated growth of Escherichia coli and Enterococcus faecalis cells.

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